The present invention relates generally to vacuum-operated sewage control systems utilizing inlet vacuum valves, and more specifically to such a system employing an electric air admission controller ("EAAC") for opening the vacuum valve independently of the level of sewage accumulated in a holding tank at the inlet of the valve, in order to introduce additional air at atmospheric pressure into the sewage transport conduit to avoid a waterlogged state.
An operational vacuum sewage transport system requires that each sewage inlet point, typically serving one or more houses, include a vacuum valve and controller assembly, which allows intermittent passage of accumulated sewage into an associated transportation conduit network connected at the other end to a collection tank, and thereafter ultimately to a sewage treatment plant. As disclosed in U.S. Pat. No. 4,179,371, issued to B. E. Foreman et al., this conduit is typically laid with a saw-toothed profile with a combination of a riser, low point, and downslope (collectively called a "lift") repeated throughout the length of the sewer main to accommodate the topography (e.g., other conduits and rock layers), as well as incoming flows (from an individual vacuum valve or a branch main). The slope of the downslope portions of the profile is such that the drop between lifts is generally equivalent to at least 40% of the conduit diameter (80% if the diameter is smaller than 6") or 0.2% of the distance between lifts, whichever is greater. Generally, the transport conduit network is continuously maintained under vacuum or subatmospheric pressure. Sewage and air, usually at atmospheric pressure, are introduced for transport into the conduit through an open vacuum valve. The air moves down the length of the conduit to the area under vacuum or subatmospheric pressure where the air expands volumetrically. The energy created by the rapid movement of air in response to the differential pressure condition in the conduit in turn produces rapid sewage transport throughout the conduit system.
At a predetermined point in time, however, the vacuum valve will close, thereby ending the sewage transport cycle. The expansion of air causes a reduction in its pressure and velocity, and any residual waste not transported through the conduit network during the sewage transport cycle comes to rest. The conduit downstream of the vacuum valve is equalized by the source of vacuum pressure to a substantially constant subatmospheric or vacuum pressure condition throughout. Any residual waste not transported through the conduit during the sewage transport cycle will generally come to rest in the low point portion, permitting vacuum or subatmospheric pressure to be communicated and maintained throughout the entire conduit section.
Vacuum valves function within this system by sealing and unsealing the passage between two parts of an evacuated system to define a transport cycle. The general structure and method of operation of this type of vacuum valve is described in U.S. Pat. No. 4,171,853, issued to D. D. Cleaver et al.
A problem encountered by vacuum transport sewage systems is "waterlogging." As already discussed, the collection of residual waste material in the low point portions of the conduit, under normal operating conditions, is insufficient to seal the conduit at low points, and is designed to maintain throughout the conduit an air space in the conduit to permit pressure communication. During a sewage transport cycle, the total conduit volume will typically be less than one-third liquid. However, if an insufficient amount of atmospheric air is introduced into the conduit, there will be insufficient energy applied to move effectively the entire waste mass during the sewage transport cycle. This leads to an increased accumulation of residual waste material, creating the waterlogged condition that might fill two-thirds of the conduit and lift volumes.
There are a number of potential causes of waterlogging. For example, the valve assembly may be misadjusted so that the valve closes too quickly after the waste material has entered the conduit, thereby undesirably reducing the amount of atmospheric air entering the conduit and pulling the waste material along. Likewise, leakage somewhere in the system will impair the maintenance of vacuum or subatmospheric pressure in the conduit so that, over time, this vacuum or subatmospheric pressure condition will decrease to the point that the differential pressure during transport cycles will be insufficient to move waste product, resulting in waterlogging. Also, while the conduit network will equalize to a vacuum or subatmospheric pressure after the valve closes, terminating the sewage transport cycle, it will do so at a slightly lower vacuum pressure due to inefficiency in the system or improper operation of the source of vacuum or subatmospheric pressure. This, in turn, contributes to a deficient level of vacuum or subatmospheric pressure and, therefore, pressure differential.
While waterlogging theoretically may occur no matter how many feet of lift are in the conduit line, the probability will increase as the "total lift" is increased, because it will be more difficult for gravity fall and the vacuum to lift sewage within the conduit lines at each succeeding profile change. Assuming that each saw-toothed lift consists of a lower downslope portion, a riser, and an upper downslope portion, the pertinent distance is measured vertically between the point on the bottom exterior surface of the upper downslope portion where it joins the riser, and the point on the top exterior surface of the lower downslope portion where it joins the riser. Aggregating these distances across the lifts of the flow path produces a measurement for the "total lift."
Typically, a vacuum system operates within a range of 16" Hg to 20" Hg vacuum. Because atmospheric pressure, on the other hand, is defined as 0" Hg in this vacuum pressure scale, this represents "a pressure differential" of 16" Hg to 20" Hg also. Taking the minimum available vacuum level of 16" Hg, and subtracting 5" Hg which must be present at all times to operate the vacuum valves and their controls leaves 11" Hg vacuum available for vacuum lift in the mains. Eleven inches of mercury is equivalent to 12.5 feet of water, which is typically rounded up to 13 feet. Thus, 13 feet of lift is typically the maximum figure used in the design of the vacuum mains for any sewer project.
Total lift of approximately 13 feet is important for two reasons. First, any system with less than 13 feet of lift which waterlogs can theoretically correct itself over time through normal valve cycling to purge the accumulated residual sewage from the conduit line. By contrast, a waterlogged vacuum transport system designed with 13 feet or more of total lift traditionally has needed operator assistance to purge the residual sewage. Because the valve opens in response to differential pressure based upon the vacuum pressure condition in the conduit immediately downstream of the valve, if that vacuum pressure is too low due to waterlogging blockage of the conduit, the valve will not cycle to introduce atmospheric pressure into the conduit, thereby preventing the conduit from automatically unwaterlogging itself over time. Instead, the repairman will have to restore the source of vacuum pressure in the system, move upstream to a valve having adequate vacuum pressure able to be activated and activate that valve, and then progressively activate each valve further upstream until the vacuum mains are cleared of the waterlogged sewage.
The second important aspect of the 13-foot measurement is that it presents a limitation on the overall length of the sewage transport lines. Combination of the predetermined slope of the lines with a maximum total lift of 13 feet determines the maximum distance the vacuum lines may travel to ensure proper sewage flow without the aid of mechanical pumps. Actually, the total permitted lift across the flow path is limited additionally by a frictional loss factor calculated according to various formulae known in the art of fluid dynamics.
In order to operate effectively a vacuum sewage system at loss levels exceeding about 13 feet, a higher air-to-liquid ratio is used. This may be simply accomplished by admitting more air into the conduit. Typical systems operating at or below the 13-foot level may be designed at a 3-to-1 air-to-liquid ratio. This number can be proportionately increased to increase lift within the system.